1. Field of the Invention
The present invention relates to an ink jet apparatus having a stable and high discharge speed.
2. Related Background Art
In an ink jet printer which supplies an electrical signal to an electro-mechanical transducer to discharge ink, an on-demand type ink jet printer which discharges ink only when an input signal is applied has been known.
A physical construction of major portions of such an ink jet printer is shown in FIG. 5.
In FIG. 5 numeral 1 denotes a platen which is rotated by a line feed pulse motor 7 to feed a record sheet, not shown. The presence or absence of the record sheet is detected by a sheet sensor 9. Numeral 2 denotes an ink jet recording head which has a plurality of ink discharge nozzles 30, is provided on a carriage slidably arranged on a guide bar, not shown and is movable along the platen 1 through a belt 14 by the drive of a D.C. carriage motor 8. In order to detect the position of the recording head 2, a linear encoder 12 and an encoder sensor 13 are provided, and in order to detect a home position, a home position sensor 11 is provided. In order to recover failure of ink discharge by the nozzles 30 of the recording head 2. an ink absorbing cap 3 which is movable by the drive of an auto-cap motor 6 is provided and the position of the cap 3 is detected by a cap sensor 10.
The control of the ink jet printer of such a construction is effected by a known CPU 20 in a control system shown in FIG. 1. The CPU 20 effects the following control operations in accordance with control inputs applied by switches 21 on a console panel, not shown. It refers inputs from the encoder sensor 13 and the home position sensor 11 to drive a carriage motor 8 through a DC servo forward/backward switching circuit 22 and drive a line feed motor 7 through a pulse motor drive circuit 23, and reads out data from a record data memory D to get a drive frequency as shown in FIG. 4 and supplies it to a head driver 24 by which the recording head 2 is driven. It also controls other units, not shown, in accordance with inputs from other sensors.
When a print switch of the switches 21 is depressed, the record operation is started. The presence of the record sheet is detected by the sheet sensor 9 and the line feed motor 7 is driven by several steps so that the platen 1 is rotated one revolution and the record sheet is set at a record start position. Then, the carriage motor 8 is driven so that the recording head 2 is reciprocally driven, and the line feed motor 7 is driven in synchronism therewith to feed the record sheet line by line. During this period, a drive signal is applied to the recording head 2 from the head driver 24 in accordance with record data so that the recording head 2 is driven and ink droplets d are discharged from the recording head 2 and characters or images are recorded on the record sheets.
A relationship between a drive frequency f discharge of the recording head 2 of the ink and a discharge speed v of the ink is shown in FIG. 4.
In order to reduce a print time, the drive frequency f may be raised. But, as seen from FIG. 4, the discharge speed v changes as the drive frequency f rises.
As shown in FIG. 4, the characteristic of the drive frequency f has a region I in which the discharge speed v does not change with the drive frequency f and a region II in which the discharge speed v changes with the drive frequency f. In order to attain a high quality of print, a drive frequency f.sub.n which meets relationships of ##EQU1## where f.sub.1 is a reference frequency equal to a maximum in the region I and n is an integer equal to or larger than 2, is used.
Accordingly, when the drive frequency f is to be increased, the characteristic of FIG. 4 is determined by a structure of the recording head 2, a property of material of the ink and a head drive waveform.
Where the region II in which the discharge speed v is not constant is used, a deviation l from the ink discharge point changes to l' when the ink discharge speed v changes to v + .DELTA.v, and an ink deposit point is deviated by ##EQU2## where L is a distance between the record medium and the record head 2 and V is a main scan speed of the record head 2.
Further, as the discharge speed v changes, a volume of ink droplet changes and there is a correlation between those two.
For this reason, when the region in which the ink discharge speed v changes with the drive frequency f is used, the quality of print is degraded.